Giovanni Battista Piranesi
Piranesi was one of the Godfathers of fantastic architecture. While much of his work was taken from real world locations, he went on to create dreamlike imaginary cities and colossal interior structures which defined a vast and incredible world. This gallery has compiled over 1,000 high quality scans of Piranesi's insanely detailed artwork. The gallery is overwhelming and easy to get lost in, much like Piranesi's imagery.
I suggest checking out his Prison series of etchings, which can be found under volume 8, images 34 - 49.